The present disclosure relates to semiconductor devices, particularly to a semiconductor device including a nitride semiconductor transistor which is applicable to power transistors used in power supply circuits in consumer products.
Group III nitride semiconductors are wide gap semiconductors. For example, band gaps of gallium nitride (GaN) and aluminum nitride (AlN) at room temperature are as large as 3.4 eV, and 6.2 eV, respectively. The group III nitride semiconductors have a high breakdown field, and a higher electron saturation velocity than arsenic semiconductors such as gallium arsenide (GaAs), etc., and semiconductors such as silicon (Si), etc. For these reasons, researches and developments have been in progress to use field effect transistors (FETs) using GaN-based nitride semiconductors as high frequency electronic devices or high output electronic devices.
The GaN-based nitride semiconductors can provide various types of mixed crystals with AlN or indium nitride (InN), and a heterojunction can be formed like the conventional arsenic semiconductors, such as GaAs. In a heterostructure using the GaN-based nitride semiconductor, e.g., an AlGaN/GaN heterostructure, high concentration carriers are generated at a heterointerface by spontaneous polarization and piezoelectric polarization even when impurities are not doped. Thus, an FET using the GaN-based nitride semiconductor tends to become a depression (normally-on) FET, and hardly becomes an enhancement (normally-off) FET. However, normally-off devices are generally used in the field of power electronics, and the devices using the GaN-based nitride semiconductor are required to be normally-off.
The normally-off transistor can be achieved in the following manner. As a first example, part of an AlGaN layer below a gate electrode in an AlGaN/GaN heterostructure is thinned to form a recess. This can reduce a concentration of two-dimensional electron gas (2DEG), and can shift a threshold voltage of the transistor to positive, thereby providing the normally-off transistor. As a second example, a {11-20}-oriented GaN layer is grown on a {10-12}-oriented principal surface of a sapphire substrate to prevent polarization field in a direction perpendicular to the principal surface of the sapphire substrate. This can provide the normally-off transistor. A minus sign attached to each miller index of the plane direction indicates inversion of an index following the minus sign.
As a promising structure for achieving the normally-off FET, a junction field effect transistor (JFET) including a p-type AlGaN layer formed in a region for forming a gate electrode has been proposed. In this JFET, the p-type AlGaN layer is connected to an AlGaN barrier layer to increase potential energy of the AlGaN barrier layer and a GaN channel layer. This can reduce the concentration of the two-dimensional electron gas generated below the region for forming the gate electrode. Thus, the JFET can be normally-off.
Another normally-off transistor capable of providing a sufficiently high current density is proposed by Japanese Patent Publication No. 2006-339561.